You can now download the March Pixel Watch update using that usual trick

The Google Pixel Watch’s March update is now available for download, bringing with it a host of new features and improvements. The update includes a new Always-On mode, which will keep the watch display active even when the battery is low, and a new “Do Not Disturb” mode that will silence all incoming notifications.
The update also brings with it a number of improvements to the watch’s fitness tracking features, including the ability to track your heart rate and exercise more accurately. Additionally, the update introduces a new suite of watch faces, including a number of options that feature Google Maps data.
As always, the update is available for download through the Google Play Store.